1. Field of the Invention
This invention relates to a cycle changeable engine capable of changing an engine operation to two-cycle operation or four-cycle operation by making opening/closing control of intake/exhaust valves by electromagnetic force.
2. Description of the Prior Art
Conventionally, a valve mechanism which constitutes electrically intake/exhaust valves and acutates them electrically as a valve mechanism of an internal combustion engine is disclosed, for example, in Japanese Patent Laid-Open No. 183805/1983. The valve mechanism of this internal combustion engine includes a detector for detecting the operational condition of the internal combustion engine and actuates the intake and exhaust valves on the basis of the detection signal of the detector. The detector for detecting the operational conditions of the internal combustion engine is accomplished by detecting a rotation state of a crank shaft or by detecting an accelerator opening. The operation of the intake and exhaust valves by the electrical motion means is accomplished by regulating the opening degree of the valve portions in response to the operational conditions of the internal combustion engine such as the intake state of a fuel and an exhaust state of an exploded and burnt matter.
In the operations of gasoline engines and Diesel engines in general, the cycle of the four operations, i.e., an intake stroke, a compression stroke, a combustion stroke and an exhaust stroke, is repeated. Among them, it is only the combustion stroke where power is generated and torque is applied to a crank shaft. In the other three strokes, rotation is effected by the force of inertia. Two kinds of systems, that is, a four-cycle engine and a two-cycle engine, exist as the operation principle of each of these strokes.
The four-cycle engine is the one that requires four strokes of the piston or in other words, two revolutions of the crank shaft in order to effect the four operations described above for each stroke of the piston and to complete one cycle. Accordingly, in the four-cycle engine, one stroke or a sufficient time, is given for the exhaust stroke and the intake stroke so that volume efficiency is high, mean effective pressure is high and efficiency is particularly high in a high speed range. Moreover, since one combustion stroke is carried out per two revolutions of the crank shaft, thermal load to each portion can be reduced. However, a valve mechanism is complicated mechanically and since the number of times of explosion is small, the operation becomes unsmooth if the number of cylinders is small and sufficient torque cannot be secured easily in a low speed range.
In contrast, in the two-cycle engine, the intake stroke and the exhaust stroke are effected at part of the combustion stroke and the compression stroke and one cycle is completed in the two strokes of the piston or in other words, one revolution of the crank shaft. Accordingly, since the number of combustion strokes or the number of times of explosion is twice in the two-cycle engine in comparison with the four-cycle engine, the output becomes about 1.5 times that of the four-cycle engine with the same exhaust quantity. However, since explosion occurs in each cycle in the two-cycle engine, there exists the problem of thermal durability of each component of the engine. Since the time necessary for scavenging in the two-cycle engine is a half of the four-cycle engine, the effective stroke is short and the loss of fresh air is great due to scavenging. It is therefore difficult to improve the mean effective pressure and engine efficiency. Moreover, blow-by is great in the two-cycle and a time area is small so that a fuel consumption quantity is greater than in the four-cycle particularly in the high speed range. Since the exhaust valves are opened earlier than in the four-cycle, an internal pressure becomes high and exhaust noise becomes great.
Demands for engines which are more compact in scale and have higher output will increase in future. As to engine performance, high torque is required in a low speed range but a great torque is not much required in a high speed range. Therefore, an engine which can be used most conveniently can be provided if the engine is constituted so that the torque in the low speed range becomes about twice the torque in the medium speed range. However, an engine having a high torque cannot be produced so easily. If a highly supercharged engine is constituted in order to produce the high torque engine, an excessive load will be applied to the engine main body and a connecting rod, a piston, and the like, will be broken by the force of the mechanical load.
The thermal load to the engine increases with the increase in the number of revolutions of engine. In an engine having a rated number of revolutions of 6,000 rpm, for example, the thermal load becomes six times the thermal load at the time of 1,000 rpm.